As a positioning system that determines the present position or the velocity of a mobile body, up to now, a global positioning system (GPS) has been used. The GSP has been widely used in not only air or vessel navigation but also a car navigation system. Also, apart from the GPS, there have been known a positioning system such as a global orbiting navigation satellite system (GLONASS), which has been developed and put into practice in Russia, and a Galileo that has been developed and put in practice by International Cooperation centered on European Union. For example, the setting of an artificial noise (PN code) or a carrier frequency used in the spread spectrum modulation of a position signal, which is broadcasted from the positioning satellite, is different between the GPS and the Galileo. However, the general positioning principle and the positioning calculating manner are similar to each other. Accordingly, a receiving device that is capable of sharing those plural positioning systems has been actively developed.
As the receiving device that is capable of sharing the plural positioning system, as disclosed in JP 7-128423A, there has been proposed a GPS/GLONASS sharing receiving device that is capable of receiving the position signals of two frequencies made up of a position signal from a positioning satellite (GPS satellite) that constructs the GPS and a position signal from a positioning satellite (GLONASS satellite) that constructs the GLONASS. The receiving device frequency-converts the respective position signal of two frequencies which have been received from the GPS satellite and the GLONASS satellite from RF (higher harmonic wave) signals to IF (intermediate frequency) signals by means of an initial-stage image removal mixer, and separates the respective position signals from each other to suppress an interference of the respective position signals, thus realizing the sharing of the GPS and the GLONASS.
In the GPS, with “GPS modernization project”, the GPS satellites of the formats called “block IIR-M” and “block IIF” have been launched in recent years, and a signal of L2C (after Block IIR-M) and a signal of L5 (after block IIF) start to be broadcasted as new consumer signals in addition to L1 that has been broadcasted for consumer use by a GPS satellite of the formats of from the beginning to Block IIR. As a result, in the GPS, the position signals of three frequencies made up of L1, L2C, and L5 are available as the consumer signals.
Also, in the Galileo, the position signals of plural frequency bands such as E2-L1-E1 (L1), E5a, E5b, and E6 are scheduled to be available as the consumer signals from the beginning. When those plural position signals can be used, it is possible to realize the performance (expansion of a received area, an improvement in the positioning precision) higher than the conventional performance.
However, the receiving device disclosed in JP 7-128423A accepts the reception of only two frequencies made up of the GPS and the GLONASS, and cannot receive the position signals of three frequencies such as L1, L2C, and L5 of the GPS. Also, for example, when an ionospheric delay error which causes the deterioration of the positioning precision is corrected by the aid of the position signals of the plural frequencies to realize the positioning with high precision, there is the possibility that the position signals cannot be received in the reception of two frequencies at the same time depending on the operational status or the arrangement status of the satellite. As a result, because a time zone during which the high-precision positioning can be realized by correction is restricted, the positioning is insufficient.
On the contrary, when the position signals of three frequencies are allowed to be received, it is possible to surely conduct higher-precision positioning than the two-frequency reception. Also, when the carrier wave positioning that can obtain the high-precision positioning result is conducted by receiving the position signals of the three frequencies, it is advantageous in that integer ambiguity can be readily determined according to an extra wide lane (EWL) method.
However, in order to accept the reception of the position signals of three frequencies, a processing system that accepts the new frequency and bandwidth is required. More specifically, for example, there are required the same number of signal processing systems as the number of received position signals. Each of the signal processing systems includes a mixer portion that converts a position signal in frequency, a local oscillator used for frequency conversion, a reference oscillator, or an amplifier portion that amplifies an intermediate wave. The above processing system suffers from such a problem that the scale of the receiving device is increased. Also, when the receiving device is increased in the scale, there occurs a problem such as an increase in power consumption or an increase in the manufacture costs.
Also, for example, because two position signals are separated by the initial-stage image removal mixer, two processing systems are required in stages downstream of the initial stage. When this structure is applied to three-frequency wave reception, the receiving device is not sufficiently downsized.